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SUBROUTINE ZLAIC1( JOB, J, X, SEST, W, GAMMA, SESTPR, S, C )
* * -- LAPACK auxiliary routine (version 3.3.1) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * -- April 2011 -- * * .. Scalar Arguments .. INTEGER J, JOB DOUBLE PRECISION SEST, SESTPR COMPLEX*16 C, GAMMA, S * .. * .. Array Arguments .. COMPLEX*16 W( J ), X( J ) * .. * * Purpose * ======= * * ZLAIC1 applies one step of incremental condition estimation in * its simplest version: * * Let x, twonorm(x) = 1, be an approximate singular vector of an j-by-j * lower triangular matrix L, such that * twonorm(L*x) = sest * Then ZLAIC1 computes sestpr, s, c such that * the vector * [ s*x ] * xhat = [ c ] * is an approximate singular vector of * [ L 0 ] * Lhat = [ w**H gamma ] * in the sense that * twonorm(Lhat*xhat) = sestpr. * * Depending on JOB, an estimate for the largest or smallest singular * value is computed. * * Note that [s c]**H and sestpr**2 is an eigenpair of the system * * diag(sest*sest, 0) + [alpha gamma] * [ conjg(alpha) ] * [ conjg(gamma) ] * * where alpha = x**H * w. * * Arguments * ========= * * JOB (input) INTEGER * = 1: an estimate for the largest singular value is computed. * = 2: an estimate for the smallest singular value is computed. * * J (input) INTEGER * Length of X and W * * X (input) COMPLEX*16 array, dimension (J) * The j-vector x. * * SEST (input) DOUBLE PRECISION * Estimated singular value of j by j matrix L * * W (input) COMPLEX*16 array, dimension (J) * The j-vector w. * * GAMMA (input) COMPLEX*16 * The diagonal element gamma. * * SESTPR (output) DOUBLE PRECISION * Estimated singular value of (j+1) by (j+1) matrix Lhat. * * S (output) COMPLEX*16 * Sine needed in forming xhat. * * C (output) COMPLEX*16 * Cosine needed in forming xhat. * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ZERO, ONE, TWO PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0, TWO = 2.0D0 ) DOUBLE PRECISION HALF, FOUR PARAMETER ( HALF = 0.5D0, FOUR = 4.0D0 ) * .. * .. Local Scalars .. DOUBLE PRECISION ABSALP, ABSEST, ABSGAM, B, EPS, NORMA, S1, S2, $ SCL, T, TEST, TMP, ZETA1, ZETA2 COMPLEX*16 ALPHA, COSINE, SINE * .. * .. Intrinsic Functions .. INTRINSIC ABS, DCONJG, MAX, SQRT * .. * .. External Functions .. DOUBLE PRECISION DLAMCH COMPLEX*16 ZDOTC EXTERNAL DLAMCH, ZDOTC * .. * .. Executable Statements .. * EPS = DLAMCH( 'Epsilon' ) ALPHA = ZDOTC( J, X, 1, W, 1 ) * ABSALP = ABS( ALPHA ) ABSGAM = ABS( GAMMA ) ABSEST = ABS( SEST ) * IF( JOB.EQ.1 ) THEN * * Estimating largest singular value * * special cases * IF( SEST.EQ.ZERO ) THEN S1 = MAX( ABSGAM, ABSALP ) IF( S1.EQ.ZERO ) THEN S = ZERO C = ONE SESTPR = ZERO ELSE S = ALPHA / S1 C = GAMMA / S1 TMP = SQRT( S*DCONJG( S )+C*DCONJG( C ) ) S = S / TMP C = C / TMP SESTPR = S1*TMP END IF RETURN ELSE IF( ABSGAM.LE.EPS*ABSEST ) THEN S = ONE C = ZERO TMP = MAX( ABSEST, ABSALP ) S1 = ABSEST / TMP S2 = ABSALP / TMP SESTPR = TMP*SQRT( S1*S1+S2*S2 ) RETURN ELSE IF( ABSALP.LE.EPS*ABSEST ) THEN S1 = ABSGAM S2 = ABSEST IF( S1.LE.S2 ) THEN S = ONE C = ZERO SESTPR = S2 ELSE S = ZERO C = ONE SESTPR = S1 END IF RETURN ELSE IF( ABSEST.LE.EPS*ABSALP .OR. ABSEST.LE.EPS*ABSGAM ) THEN S1 = ABSGAM S2 = ABSALP IF( S1.LE.S2 ) THEN TMP = S1 / S2 SCL = SQRT( ONE+TMP*TMP ) SESTPR = S2*SCL S = ( ALPHA / S2 ) / SCL C = ( GAMMA / S2 ) / SCL ELSE TMP = S2 / S1 SCL = SQRT( ONE+TMP*TMP ) SESTPR = S1*SCL S = ( ALPHA / S1 ) / SCL C = ( GAMMA / S1 ) / SCL END IF RETURN ELSE * * normal case * ZETA1 = ABSALP / ABSEST ZETA2 = ABSGAM / ABSEST * B = ( ONE-ZETA1*ZETA1-ZETA2*ZETA2 )*HALF C = ZETA1*ZETA1 IF( B.GT.ZERO ) THEN T = C / ( B+SQRT( B*B+C ) ) ELSE T = SQRT( B*B+C ) - B END IF * SINE = -( ALPHA / ABSEST ) / T COSINE = -( GAMMA / ABSEST ) / ( ONE+T ) TMP = SQRT( SINE*DCONJG( SINE )+COSINE*DCONJG( COSINE ) ) S = SINE / TMP C = COSINE / TMP SESTPR = SQRT( T+ONE )*ABSEST RETURN END IF * ELSE IF( JOB.EQ.2 ) THEN * * Estimating smallest singular value * * special cases * IF( SEST.EQ.ZERO ) THEN SESTPR = ZERO IF( MAX( ABSGAM, ABSALP ).EQ.ZERO ) THEN SINE = ONE COSINE = ZERO ELSE SINE = -DCONJG( GAMMA ) COSINE = DCONJG( ALPHA ) END IF S1 = MAX( ABS( SINE ), ABS( COSINE ) ) S = SINE / S1 C = COSINE / S1 TMP = SQRT( S*DCONJG( S )+C*DCONJG( C ) ) S = S / TMP C = C / TMP RETURN ELSE IF( ABSGAM.LE.EPS*ABSEST ) THEN S = ZERO C = ONE SESTPR = ABSGAM RETURN ELSE IF( ABSALP.LE.EPS*ABSEST ) THEN S1 = ABSGAM S2 = ABSEST IF( S1.LE.S2 ) THEN S = ZERO C = ONE SESTPR = S1 ELSE S = ONE C = ZERO SESTPR = S2 END IF RETURN ELSE IF( ABSEST.LE.EPS*ABSALP .OR. ABSEST.LE.EPS*ABSGAM ) THEN S1 = ABSGAM S2 = ABSALP IF( S1.LE.S2 ) THEN TMP = S1 / S2 SCL = SQRT( ONE+TMP*TMP ) SESTPR = ABSEST*( TMP / SCL ) S = -( DCONJG( GAMMA ) / S2 ) / SCL C = ( DCONJG( ALPHA ) / S2 ) / SCL ELSE TMP = S2 / S1 SCL = SQRT( ONE+TMP*TMP ) SESTPR = ABSEST / SCL S = -( DCONJG( GAMMA ) / S1 ) / SCL C = ( DCONJG( ALPHA ) / S1 ) / SCL END IF RETURN ELSE * * normal case * ZETA1 = ABSALP / ABSEST ZETA2 = ABSGAM / ABSEST * NORMA = MAX( ONE+ZETA1*ZETA1+ZETA1*ZETA2, $ ZETA1*ZETA2+ZETA2*ZETA2 ) * * See if root is closer to zero or to ONE * TEST = ONE + TWO*( ZETA1-ZETA2 )*( ZETA1+ZETA2 ) IF( TEST.GE.ZERO ) THEN * * root is close to zero, compute directly * B = ( ZETA1*ZETA1+ZETA2*ZETA2+ONE )*HALF C = ZETA2*ZETA2 T = C / ( B+SQRT( ABS( B*B-C ) ) ) SINE = ( ALPHA / ABSEST ) / ( ONE-T ) COSINE = -( GAMMA / ABSEST ) / T SESTPR = SQRT( T+FOUR*EPS*EPS*NORMA )*ABSEST ELSE * * root is closer to ONE, shift by that amount * B = ( ZETA2*ZETA2+ZETA1*ZETA1-ONE )*HALF C = ZETA1*ZETA1 IF( B.GE.ZERO ) THEN T = -C / ( B+SQRT( B*B+C ) ) ELSE T = B - SQRT( B*B+C ) END IF SINE = -( ALPHA / ABSEST ) / T COSINE = -( GAMMA / ABSEST ) / ( ONE+T ) SESTPR = SQRT( ONE+T+FOUR*EPS*EPS*NORMA )*ABSEST END IF TMP = SQRT( SINE*DCONJG( SINE )+COSINE*DCONJG( COSINE ) ) S = SINE / TMP C = COSINE / TMP RETURN * END IF END IF RETURN * * End of ZLAIC1 * END |